This invention relates to lasers and more particularly to laser oscillator/amplifiers using phase conjugate reflectors to compensate for wavefront distortions.
Lasers and laser systems suffer from several sources of wavefront distortion and spectral broadening. Effects caused by thermal stress or gradients, vibration, or aberrations are all problems that contribute to distortions in the radiation wavefronts traversing a laser. In addition, thermal effects cause large transient effects on the operation during start-up, before the thermal equilibrium of steady state operation is reached for the optical elements. This can be a severe problem for pulsed lasers and CW lasers if they are operated in bursts, since the transient period then comprises a larger percentage of the overall laser operating cycle.
Wavefront distortions degrade the performance of a laser and manifest themselves as poor energy extraction efficiency and beam divergence. Depending on the output beam quality and power levels desired, wavefront distortion in the components internal to the laser can make it inadequate for a given application. This is especially true for advanced communications or tracking systems where a nearly diffraction limited laser output is desired. Moreover, even if steady state operation can be made to be satisfactory by conventional means, thermal effects in high power or high energy density lasers still create unsatisfactory performance during the initial start-up periods prior to steady state operation, when the laser elements are acquiring thermal equilibrium.
Several techniques have been proposed for removing or preventing wavefront distortion including specialized retro-reflectors and deformable mirrors. In typical solid state laser rods, thermal gradients generated by the pumping energy causes an effect known as positive lensing which can be corrected with a negative lens. However, for the transient state mechanical drives must be employed to dynamically compensate for the changing lensing. While this has been employed with some success, there are several drawbacks. First, for geometries that are not radially symmetric, such as slab geometry laser gain media, media lensing effects are not those of a simple lens and requires complex lens structures to compensate. Second, any "dynamic" lens system requires precise closed-loop detection and adjustment at very high speeds. This has led to the development of electronically controlled, mechanically driven, deformable or flexible mirrors and feedback loops or servo systems in large laser systems. As is readily apparent, this makes a laser or laser system more complex, costly and alignment error prone than desired. This approach also still has operating response (speed) limitations which simply cannot match the transient effects rate to adequately compensate for a variety of applications. No mechanical system has achieved the desired level of compensation for advanced, nearly diffraction limited lasers.
In addition, many applications require or desire efficient operation even during the initial warm-up or start-up phase of operation. Even the previously proposed elaborate mechanical thermal lensing compensation systems have failed to achieve good performance, in terms of efficiency, under transient warm-up conditions. Diffraction limited performance has been demonstrated only in laboratory lasers under steady state conditions.
An alternate proposal to aberration compensation in lasers is the use of phase conjugating reflectors. Here laser energy from a gain medium is reflected by a phase conjugate reflector, which replaces a conventional laser mirror, and then passed back through the gain medium in a double pass configuration.
The phase conjugate reflector produces reflected radiation whose wavefronts behave as a time reversed version of the incident wavefronts. This allows the radiation to re-traverse the same optical path, including any aberrations, in a reverse sense and thus become a non-distorted wavefront. This is discussed in more detail in "Optical Phase Conjugation" by V. V. Shkunov and B. Y. Zel'dovich in SCIENTIFIC AMERICAN 253 #6, 54 (December 1985). In this approach phase conjugation would be used to passively compensate for short and long term transient response or behavior as well as compensate for steady state optical distortions. However, the proposed phase conjugation schemes or embodiments described in the literature either have not been demonstrated, require relatively large amounts of oscillator power and are consequently relatively inefficient, or do not address limitations on energy extraction imposed by amplified spontaneous emission in the amplifying medium.
The basic premise of the oscillator/amplifier arrangement is as follows. Oscillators with excellent beam quality and transient performance can be made at low power. As oscillators are scaled to higher power, thermal problems in the lasing medium become progressively more severe, making it progressively more difficult to simultaneously obtain good oscillator quality, transient performance, and efficiency. The same problems occur with amplifiers; however with amplifiers these problems can more readily be surmounted by the use of phase conjugation, or other, techniques. Therefore, in an oscillator/amplifier it is desirable to minimize the fraction of the total output, and input, energy in the oscillator.
Laser oscillator/amplifiers using phase conjugation have been described in the Soviet literature in articles such as "Cancellation of Phase Distortions in an Amplifying Medium with a Brillouin Mirror," O. Y. Nosach, et al, ZhETF PIS. RED., Vol. 16, No. 11, pp. 617-621 (Dec. 5, 1972) and "Connection between the Wave Fronts of the Reflected and Exciting Light in Stimulated Mandel'Shtam-Brillouin Scattering," B. Y. Zel'dovich, et al, ZhETF PIS. RED., Vol. 15, No. 3, pp. 160-164 (Feb. 5, 1972). Although pulsed systems operating at a high energies have been described, those operating at a high repetition rate have been low gain and energy.
Additional lasers or laser systems are illustrated in U.S. Pat. Nos. 4,321,550 to Evtohov, and 4,233,571 to Wang et al. These patents cover lasers whose output is obtained after single or multiple passing laser radiation through a gain medium. Energy is extracted out of the gain media on one round trip. While these lasers appear to offer aberration compensation and improved wavefront output, they require relatively large amounts of oscillator energy to drive since the medium is not efficiently driven to saturation. Also, these designs do not allow for a wide range of optical distortions in the lasing medium, and, therefore, transient performance is not good and/or performance is limited to a narrow power range.
What is needed, then, is a method to saturate the laser gain media, and thus obtain high extraction efficiency while minimizing oscillator performance requirements. This requires that the amplifier has high gain. However, since the gain is limited by amplified spontaneous emission, methods are also needed to minimize this effect. At the same time, methods are needed to improve the optical layout so that phase conjugation can be made effective over a wide range of optical distortions, allowing operation during transients and over a wide power range.